We present explicit expressions for the linear and the second nonlinear imaginary parts of admittanc (emittance) for the charge polarization of accumulation on both sides of the quantum dot (cavity) junctions by using Green function and the coupling parameters in an effective Hamiltonian and the discrete potential model. We found that the emittance and the electrochemical capacitance are equal to the geometric capacitance in the classical limit. In the nonclassical case the emittance is equal to the electrochemical capacitance, but not equal to the geometric capacitance if there is complete reflection. In the case where there is tunneling the emittance and electrochemical capacitance as well as the geometric capacitance are different. The results may be helpful for measurements on capacitance on small-scale structures.